In recent years, high-density mounting of electrical circuits has been widely used as equipment is miniaturized and becomes electronic. Accordingly, because of the increase in heat density of the electronic equipment, a blower has been used for cooling the equipment.
For a conventional blower, as shown in FIG. 9, an annular wall 2 is formed so as to be spaced from the blade tip end of an axial fan 1, and the axial fan 1 rotates around a shaft 4 in a blowing state in which a motor 3 is energized, by which an air flow (air quantity per unit time) 5 directed from the suction side to the discharge side is generated.
However, in the aforementioned blowing state, the velocity of the air flow increases on the back pressure side of the blade tip end, and a low-energy zone due to the effect of an inter-blade secondary flow is produced on the blade trailing edge side where the air flow is converted into pressure energy. In this zone, loss is great, and the flow is liable to be separated. The air flow is separated from the blade surface, so that vortexes are produced in the separation region, by which turbulence noise is increased, and the noise level and the air flow vs. static pressure characteristics deteriorate.
This phenomenon is frequently found especially when a flow resistance (system impedance) is applied to the discharge flow side and when the occurrence of leakage vortexes at the blade tip end increases, by which the fan goes into a stalling state.
To solve this problem, in a blower disclosed in a prior application (Japanese Patent Application Laid-Open No. 10-18995) filedby the same applicant as that of this invention, a method has been disclosed in which air is sucked to the inside of the annular wall through slits formed in the annular wall in the blowing state to restrain leakage vortexes at the blade tip end and rotating stall, by which the air flow vs. static pressure characteristics are improved and the noise is reduced.
FIGS. 10a to 10c show a blower as disclosed in Japanese Patent Application Laid-Open No. 10-18995.
For this blower, slits 6 are formed in an annular wall 2 surrounding an axial fan 1. Specifically, annular plates 7.sub.1, 7.sub.2, 7.sub.3, 7.sub.4, 7.sub.5 are laminated with spacers 8 being held therebetween, and a slit 6 is formed between the adjacent annular plates of the annular plates 7.sub.1, . . . 7.sub.5.
FIG. 11 shows the characteristics of this blower at the same rotational speed.
The blower with slits, in which air is allowed to flow in from the outer periphery of the annular wall 2, is superior to the conventional (no-slit) blower of the same size in terms of air flow and noise at high pressures. However, the blower with slits has a problem in that the air flow at low pressure is slightly small, and the power (a value obtained by multiplying driving torque by rotational speed, hereinafter referred to as a fan driving force) at the time when the fan is driven at the same rotational speed is high, so that the fan static pressure efficiency (a value obtained by multiplying static pressure by air flow and dividing by fan driving force) is decreased.